The present invention relates to optical functional devices and optical modules. In particular, the present invention relates to optical functional devices for controlling optical propagation (e.g., optical isolators and optical circulators), and to optical modules including the devices.
In a high-speed optical communication system, a reflected feedback light might be generated from an end face of an optical component or optical functional device, for example. This reflected feedback light is undesirable because it causes instability in the oscillation of a light source such as a semiconductor laser, which results in characteristic degradation. Therefore, an optical isolator for blocking such a feedback light is an indispensable device when an optical functional device such as a semiconductor laser or optical amplifier is used.
On the other hand, in constructing a processing system for optical communication such as bi-directional optical communication, an optical circulator that can divert and extract a light in only one propagation direction is also indispensable. Optical isolator and optical circulator exhibit optical nonreciprocity in which their output intensity characteristics are varied depending on the propagation direction of light; thus, they are utilized in various fields. Accordingly, optical isolator and optical circulator are important optical functional devices.
FIGS. 9A and 9B schematically illustrate the function of an optical isolator and that of an optical circulator, respectively. As shown in FIG. 9A, the optical isolator allows a light transmitted in one direction to pass through but blocks a light transmitted in the opposite direction. On the other hand, as shown in FIG. 9B, the optical circulator allows a light transmitted in one direction to pass through but blocks and deflects a light transmitted in the opposite direction.
FIG. 10 illustrates the configuration of a bulk-type optical isolator having the function shown in FIG. 9A.
The bulk-type optical isolator that is generally used as a device exhibiting optical nonreciprocity (hereinafter called an “optical nonreciprocity device”) includes: magnetized magnetic crystal 25 that enables the utilization of Faraday effect; and polarizers 26 that are used in combination with the magnetic crystal 25, thus allowing a light transmitted in one direction (i.e., a light transmitted in +Z direction) to pass through and blocking a light transmitted in the opposite direction (i.e., a light transmitted in −Z direction). Specifically, the polarization direction of a light incident on the magnetic crystal 25 (i.e., a Faraday rotator) is rotated 45° in accordance with the direction of magnetization, and thus the optical isolator exhibits optical nonreciprocity in which its output characteristic varies depending on the direction of the light incidence. A waveguide-type optical isolator is also provided based on the similar principle and is formed to include a magnetic material for utilizing magneto-optical effect, and polarizers used in combination with the magnetic material.
Next, FIG. 11 illustrates the configuration of a one fiber bi-directional optical module. The optical module illustrated in FIG. 11 includes an optical fiber 36, a silicon substrate 37 and a glass substrate 38. In the optical module, a photodiode for monitoring 32 and a semiconductor laser 33 are provided on the silicon substrate 37; on the other hand, a photodiode for reception 35 is provided on the glass substrate 38, and a wavelength-division filter 34 is inserted into the optical fiber 36.
In this optical module for optical communication, optical output intensity is important since it affects the quality of communication. Therefore, it is preferable that the number of components is decreased to provide a simple configuration and thus coupling loss is reduced. Furthermore, if a decrease in the number of components is enabled and thus a simple configuration can be provided, it becomes possible to achieve cost reduction in the optical module.
As for related application, reference may be made to Japanese Unexamined Patent Publication No. 7-283485 (see FIG. 3 in this publication) although it will be described later.